1. Field of the Invention
The present invention relates to mounting a focal plane array (FPA) on a supporting substrate. More particularly, the mounting structure creates controlled stresses to counteract deflections that adversely affect the reliability of the FPA in the process of thermal cycling.
2. Description of Related Art
A hybrid FPA generally consists of an optical substrate layer, such as sapphire, coated with an optically-sensitive material. An interconnecting scheme typically formed of pliable conducting bumps, is used to establish a mechanical and electrical contact between the optical substrate and an electronic means, such as a silicon multiplexer (MUX), used for electronic signal processing. A difference between the thermal expansion coefficients (TECs) of the detector substrate and the MUX, however, can stress and eventually rupture the bumps in the process of thermal cycling.
To enhance the reliability of a hybrid FPA, a shimming, sandwiched approach has been used (FIGS. 1 and 2), in which a hybrid FPA is mounted on a multi-layer substrate. This approach has been described in related patent application Ser. No. 08/409,230, entitled "HYBRID FOCAL PLANE ARRAY STABILIZING APPARATUS AND METHOD," the disclosure of which is incorporated herein. The MUX 110, 210 is bonded onto a core layer 114, 214 having a high TEC using a bonding epoxy 112, 212. The core layer is then bonded onto a balancing layer 118, 218 of the same material and dimensions as the MUX 110, 210 using the same type of epoxy of the same thickness as the bonding epoxy 112, 212. The MUX, the core layer, the two layers of epoxy, and the balancing layer form a "balanced" structure 124, 224 that forces the MUX to exhibit a TEC which matches the TEC of the optical layer 104, 204, thereby eliminating thermally-generated stresses on the interconnect bumps 108, 208. The balanced nature of such a hybrid FPA and composite structure also prevents the structure from deflecting.
However, since the FPA 100, 200 is generally operated at cryogenic temperatures, reliability concerns may arise when the FPA is turned on and cooled to the operating temperature, and subsequently warmed to ambient temperature when turned off. In FIGS. 1 and 2, the TEC of the composite structure 124, 224 matches that of the optical substrate 104, 204. However, as shown in FIG. 1, both the MUX 110 and the balancing layer 118 may be deflected toward the core layer 114 at the edges due to the edge discontinuity. The convex deflection of the MUX surface in FIG. 2 is caused by contraction of mounting epoxy 220.
Undesirable edge effect deflections could also be produced during contraction of the shimming epoxy 120 while curing, as well as during the cooling process. The convex deflection of the MUX 110 strains the edge bumps 108 leading to bump damage. A convex deflection of the MUX may occur under other circumstances as well. Referring to FIG. 2, if the composite structure 224, including the MUX 210, the core and balancing layers 214, 218 are mounted on a support surface 222, a convex deflection of the MUX and the other layers in the composite structure could also occur due to contraction of the epoxy 220 used in mounting the composite structure onto the supporting surface 222. Because epoxies generally have a higher TEC than silicon, sapphire and the core material, the mounting epoxy may have a tendency to contract with changing temperatures at a higher rate than the other layers in the structure. This contraction of mounting epoxy 220 may occur both during curing and cooling, depending upon the particular type of epoxy used.